Digitally Controlled Ring Oscillators are a type of controlled oscillator that can be implemented in a number of different ways. One common implementation includes one or more stages of tri-state inverters connected in parallel. Each stage of inverters is comprised of an odd number of tri-state inverters connected in a loop, with the output of one inverter feeding the input of the next inverter. The looped connection of the odd number of inverters creates oscillation of the inverters between two logical states at a frequency that may be adjusted by means of a digital input word, using different techniques.
In one example, the oscillation frequency may be adjusted by switching on or off one or more of the tri-state inverters connected in one or more stages of the ring. Sets of parallel-connected tri-state inverters or stages of inverters may be activated or cut out as a group to adjust the oscillation frequency in steps.
Tri-state inverters have generally been comprised of a chain of four Metal-Oxide Semiconductor (MOS) devices connected in series: two p-type Metal-Oxide Semiconductor (PMOS) devices ahead of two n-type Metal-Oxide Semiconductor (NMOS) devices. Generally, the two MOS devices in the center of the chain are the “core” devices, functioning as an inverter, while the two MOS devices on the ends perform switching functions, either energizing or cutting out the core inverter devices.
It is usually desirable that the supply of such tri-state inverters is regulated for power supply noise rejection. As a consequence, when implemented as part of an oscillator circuit, the core inverter of a tri-state inverter is generally powered by a series of two PMOS devices, one constituting the output device of a low-drop voltage regulator feeding the circuit, and the other constituting the upper enabling switch of the tri-state inverter.
Using the methods and apparatuses described below, the functions of voltage regulation and power switching can be implemented with only one device, thereby resulting in the core inverter of the tri-state inverter being powered by only one PMOS device.